Various heat transfer techniques have so far been known in the art, including a sublimation transfer system wherein a sublimable dye is carried on a substrate sheet as a recording material to form a heat transfer sheet, and that transfer sheet is then overlaid on an image-receiving material dyeable with such a sublimable dye, e.g. a polyester woven fabric to apply heat energy from the back surface of the heat transfer sheet, thereby transferring the sublimable dye onto the image-receiving material.
More recently, it has been proposed to make various full-color images on paper or plastic films, using the aforesaid sublimation type of heat transfer system. In this case, a printer's thermal head is used as heating means to apply very quick heating to transfer a number of three-, four- or multi-color dots onto an image-receiving material, thereby reconstructing a full-color image of the original image with, the multi-color dots. Because the coloring matter used is a dye, the thus formed image is very clear-cut and excels in transparency. Thus that image is improved in terms of the reproducibility of the halftone and gray scale, making it possible to form first-line image representations equivalent to those achieved by conventional offset or gravure printing and comparable to full-color photographs as well.
With the aforesaid heat transfer process, however, the most intractable problems arise in connection with how clearly the resulting image can develop colors, how long it will be well stored and how resistive it will be to discoloring and browning.
In other words, it is required for fast recording that heat energy be applied for as short a time as possible, say, on the order of milliseconds. Thus the sublimable dye and image-receiving material are not well heated within such a short time, rendering it impossible to make images of sufficient density.
In order to accommodate such fast recording, sublimable dyes having superior sulimability have thus been developed. Since dyes excelling in sublimability have generally a low molecular weight, however, they tend to migrate or bleed with time through image-receiving materials after transfer, offering storability problems such as making the resulting images blurry or unclear or causing them to foul surrounding objects.
In order to avert such a problem, it has been proposed to use sublimable dyes having a relatively high molecular weight. However, their rate of sublimation is too low for such high speed recording as mentioned above to make images of such sufficient density as aforesaid.
In addition, because the images are made from dyes, the resulting images are generally inferior in light fastness to pigmented images. In addition, they turn brown or discolor more prematurely than the pigmented images, when exposed to direct sunlight. Such light fastness problems may have been solved to some extent by adding UV absorbers or antioxidants to the dye-receiving layers of image-receiving materials.
However, the discoloring and browning problems arise by light other than direct sunlight as well. For instance, discoloring or browning are likely under indoor light or under such conditions as not directly exposed to light, e.g. in albums, cases or books. These in-room or in-the-dark discoloring or browning problems can never be solved by using general UV absorbers and antioxidants.